1. Field of the Invention
The present invention relates generally to a screw having an offset head and to a multiaxial or polyaxial screw for use with a bone fixation system and, in particular, to a multiaxial or polyaxial screw which permits a greater range of pivotability in at least one direction.
2. Description of the Related Art
As is generally known in the art, spinal abnormalities may be correctable using a pair of posterior spinal fixation rods attached to the vertebrae using pedicle screws and the like. In order to provide increased stability and rigidity, especially to resist twisting or the like, the pair of elongated rods often include cross connecting devices. The cross connecting devices typically traverse the spinal column and couple to each of the elongated rods. In other words, the cross connecting devices are perpendicular or substantially perpendicular to the spinal column.
In addition, bone screws with a polyaxial head are commonly used in spine surgery today. They are used chiefly in the lumbar spine and screwed into bone (pedicle) posteriorly. The head of the screw is attached to the shaft of the screw. The head of the screw is machined into a ball, and the head may be provided with a receiver or socket into which the ball fits. One typical prior art system further contains a receiver for receiving a separate rod. The rod is fastened to the screw head receiver via a threaded cap. The rod is then fastened to screws placed in adjacent vertebrae thus providing stabilization. The polyaxial head allows the rod to be placed in a variety of angles with respect to the screw allowing conformance to local anatomy.
Exemplary bone screws are disclosed in the following patents: U.S. Pat. No. 5,466,237 to Bird et al.; U.S. Pat. No. 4,946,458 to Harms; U.S. Pat. No. 5,207,678 to Harms et al.; U.S. Pat. No. 5,474,555 to Puno et al.; and U.S. Pat. No. 6,869,433 to Glascott. It will be appreciated from the prior art, however, that multiaxial screws involve the ability to pivot symmetrically or the same amount in each direction. In some instances, it is desired for the retainer to be moved or pivoted about the screw head more in a certain direction than in another, whereby greater maneuverability of the rod attached to the retainer may be accommodated. It was difficult to get a “favorable angle” of the retainer relative to the screw head.
FIGS. 1-3B show a prior art system wherein a retainer having an angled or canted surface that permits movement between a predetermined angle A (FIG. 2) and a different smaller angle B (FIG. 3A) when the retainer is moved between a first direction C and a second direction D, respectively.
Thus, it would be desirable for a multiaxial screw assembly to be developed for use with a spinal fixation system which permits non-symmetrical pivoting and that is capable of use with a variety of retainers, including those with non-canted surfaces. It would also be desirable for a multiaxial screw assembly to be developed in which simple modification of existing components enables the desired pivoting action.